Copper alloys used in electronic machinery and tools are given various kinds of metal plating, seeking to impart more functionality thereto. In their use for lead frames, for example, they are given Ag plating for wire bonding, Cu plating as a foundation for the said Ag plating, and solder plating for mounting on boards.
Recent years have also seen the use of plated lead frames prepared by giving metal plating throughout the surface of lead frames after forming lead by etching or presswork. The metal plating given therein includes Pd plating, and Ni plating as a foundation thereof.
In recent years, further increases in packing density and further reductions in size and profile have been required of electronic components used in electronic machinery and tools. In response to these requirements, efforts have been moving ahead to increase the number of pins and to narrow pitches in the fields of lead frames, terminals, and connectors. From the viewpoint of environmental issues, the solder used in mounting those components on boards has been required to be free of lead, and then many lead-free solders bring about a rise in soldering temperatures at the time of mounting on boards.
As pitches are narrowed, problems may be caused in the producing method of lead frames. Specifically, there may be cases in which the Ag plating given in the producing method of lead frames causes anomalous precipitation and growth to a projecting form, thereby lowering the bonding strength of wire bonding, and shorts may occur between adjacent leads.
In addition, the Cu plating as a foundation is exposed in areas to which Ag plating is not given, and when it is heated in a packaging process, an oxidation film on the lead frame surface is formed. In these areas also, anomalous precipitation, for example, porous precipitation, of the Cu plating causes formation of oxide film inferior in adhesiveness on the lead frame surface, to result in lowering of adhesion between a mold resin and lead frames. Thus, the problem occurs that, when soldering onto boards is carried out, packages become cracked in a reflowing furnace.
Seeking to resolve those these problems, the method of limiting the number of precipites in a copper alloy, which form starting points of anomalous precipitation of plating, to a certain value or below, and the method of limiting the contents of addition elements or impurity elements to certain values or below, have been proposed. However, these methods, though effective on copper alloys having specific alloy compositions, cannot be applied to other copper alloys. Under the circumstances, methods applicable to the entire copper alloys have not yet been found.
Additionally, although methods of improving the influences of the work affected layer present on the surface of a metallic material have so far been proposed, they aim to improve adhesiveness and solder wettability of the metal-plated layer, and differ in substance from the present invention based on the finding of the relationship between the anomalous precipitation of metal plating (e.g. projecting precipitation, porous precipitation) and the thickness of the work affected layer.